.\"	$OpenBSD: SSL_CTX_set_tmp_dh_callback.3,v 1.8 2020/03/30 10:28:59 schwarze Exp $
.\"	OpenSSL b97fdb57 Nov 11 09:33:09 2016 +0100
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.Dd $Mdocdate: March 30 2020 $
.Dt SSL_CTX_SET_TMP_DH_CALLBACK 3
.Os
.Sh NAME
.Nm SSL_CTX_set_tmp_dh_callback ,
.Nm SSL_CTX_set_tmp_dh ,
.Nm SSL_set_tmp_dh_callback ,
.Nm SSL_set_tmp_dh
.Nd handle DH keys for ephemeral key exchange
.Sh SYNOPSIS
.In openssl/ssl.h
.Ft void
.Fo SSL_CTX_set_tmp_dh_callback
.Fa "SSL_CTX *ctx"
.Fa "DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength)"
.Fc
.Ft long
.Fn SSL_CTX_set_tmp_dh "SSL_CTX *ctx" "DH *dh"
.Ft void
.Fo SSL_set_tmp_dh_callback
.Fa "SSL *ssl"
.Fa "DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength"
.Fc
.Ft long
.Fn SSL_set_tmp_dh "SSL *ssl" "DH *dh"
.Sh DESCRIPTION
.Fn SSL_CTX_set_tmp_dh_callback
sets the callback function for
.Fa ctx
to be used when a DH parameters are required to
.Fa tmp_dh_callback .
The callback is inherited by all
.Vt ssl
objects created from
.Fa ctx .
.Pp
.Fn SSL_CTX_set_tmp_dh
sets DH parameters to be used by
.Fa ctx .
The key is inherited by all
.Fa ssl
objects created from
.Fa ctx .
.Pp
.Fn SSL_set_tmp_dh_callback
sets the callback only for
.Fa ssl .
.Pp
.Fn SSL_set_tmp_dh
sets the parameters only for
.Fa ssl .
.Pp
These functions apply to SSL/TLS servers only.
.Pp
When using a cipher with RSA authentication,
an ephemeral DH key exchange can take place.
Ciphers with DSA keys always use ephemeral DH keys as well.
In these cases, the session data are negotiated using the ephemeral/temporary
DH key and the key supplied and certified by the certificate chain is only used
for signing.
Anonymous ciphers (without a permanent server key) also use ephemeral DH keys.
.Pp
Using ephemeral DH key exchange yields forward secrecy,
as the connection can only be decrypted when the DH key is known.
By generating a temporary DH key inside the server application that is lost
when the application is left, it becomes impossible for an attacker to decrypt
past sessions, even if he gets hold of the normal (certified) key,
as this key was only used for signing.
.Pp
In order to perform a DH key exchange the server must use a DH group
(DH parameters) and generate a DH key.
The server will always generate a new DH key during the negotiation.
.Pp
As generating DH parameters is extremely time consuming, an application should
not generate the parameters on the fly but supply the parameters.
DH parameters can be reused,
as the actual key is newly generated during the negotiation.
The risk in reusing DH parameters is that an attacker may specialize on a very
often used DH group.
Applications should therefore generate their own DH parameters during the
installation process using the
.Xr openssl 1
.Cm dhparam
application.
This application guarantees that "strong" primes are used.
.Pp
Files
.Pa dh2048.pem
and
.Pa dh4096.pem
in the
.Pa apps
directory of the current version of the OpenSSL distribution contain the
.Sq SKIP
DH parameters,
which use safe primes and were generated verifiably pseudo-randomly.
These files can be converted into C code using the
.Fl C
option of the
.Xr openssl 1
.Cm dhparam
application.
Generation of custom DH parameters during installation should still
be preferred to stop an attacker from specializing on a commonly
used group.
The file
.Pa dh1024.pem
contains old parameters that must not be used by applications.
.Pp
An application may either directly specify the DH parameters or can supply the
DH parameters via a callback function.
.Pp
Previous versions of the callback used
.Fa is_export
and
.Fa keylength
parameters to control parameter generation for export and non-export
cipher suites.
Modern servers that do not support export ciphersuites are advised
to either use
.Fn SSL_CTX_set_tmp_dh
or alternatively, use the callback but ignore
.Fa keylength
and
.Fa is_export
and simply supply at least 2048-bit parameters in the callback.
.Sh RETURN VALUES
.Fn SSL_CTX_set_tmp_dh
and
.Fn SSL_set_tmp_dh
do return 1 on success and 0 on failure.
Check the error queue to find out the reason of failure.
.Sh EXAMPLES
Set up DH parameters with a key length of 2048 bits.
Error handling is partly left out.
.Pp
Command-line parameter generation:
.Pp
.Dl openssl dhparam -out dh_param_2048.pem 2048
.Pp
Code for setting up parameters during server initialization:
.Bd -literal
SSL_CTX ctx = SSL_CTX_new();
\&...

/* Set up ephemeral DH parameters. */
DH *dh_2048 = NULL;
FILE *paramfile;
paramfile = fopen("dh_param_2048.pem", "r");
if (paramfile) {
	dh_2048 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
	fclose(paramfile);
} else {
	/* Error. */
}
if (dh_2048 == NULL) {
	/* Error. */
}
if (SSL_CTX_set_tmp_dh(ctx, dh_2048) != 1) {
	/* Error. */
}
.Ed
.Sh SEE ALSO
.Xr openssl 1 ,
.Xr ssl 3 ,
.Xr SSL_CTX_set_cipher_list 3 ,
.Xr SSL_CTX_set_options 3 ,
.Xr SSL_set_tmp_ecdh 3
.Sh HISTORY
.Fn SSL_CTX_set_tmp_dh_callback
and
.Fn SSL_CTX_set_tmp_dh
first appeared in SSLeay 0.8.0 and have been available since
.Ox 2.4 .
.Pp
.Fn SSL_set_tmp_dh_callback
and
.Fn SSL_set_tmp_dh
first appeared in OpenSSL 0.9.2b and have been available since
.Ox 2.6 .
